Method for measuring the impedance of an electrodeless arc discharge lamp

ABSTRACT

A simulated load circuit for measuring the impedance of the arc discharge of an electrodeless discharge lamp of the type having an arc tube and an excitation coil for exciting the arc discharge in an ionizable fill contained therein includes: a secondary coil spaced apart from the excitation coil by a distance which is varied in order to vary the coupling coefficient between the secondary coil and the excitation coil; a fixed load resistance coupled to the secondary coil; and a variable matching network coupled in series or parallel with the load resistance, the impedance of the matching network being varied in order to vary the ratio of reactance to resistance of the load circuit. The distance between the secondary coil and the excitation coil is varied, and the impedance of the matching network is varied, until the input impedance of the load circuit is substantially equivalent to the operating impedance of the lamp. The simulated load circuit is useful for designing and testing ballast circuits for electrodeless discharge lamps and for providing measurements of arc discharge power and excitation coil efficiency.

FIELD OF THE INVENTION

The present invention relates generally to electrodeless discharge lampsand, more particularly, to a method and circuit for measuring theimpedance of the plasma discharge of such a lamp (e.g., a high intensitydischarge lamp) and to a simulated load for the lamp ballast useful inthe production and testing thereof.

BACKGROUND OF THE INVENTION

In a high intensity discharge (HID) lamp, a medium to high pressureionizable gas, such as mercury or sodium vapor, emits visible radiationupon excitation typically caused by passage of current through the gas.One class of HID lamps comprises electrodeless lamps which generate anarc discharge by generating a solenoidal electric field in ahigh-pressure gaseous lamp fill. In particular, the lamp fill, ordischarge plasma, is excited by radio frequency (RF) current in anexcitation coil surrounding an arc tube. The arc tube and excitationcoil assembly acts essentially as a transformer which couples RF energyto the plasma. That is, the excitation coil acts as a primary coil, andthe plasma functions as a single-turn secondary. RF current in theexcitation coil produces a time-varying magnetic field, in turn creatingan electric field in the plasma which closes completely upon itself,i.e., a solenoidal electric field. Current flows as a result of thiselectric field, resulting in a toroidal arc discharge in the arc tube.

In developing high-efficiency RF circuits to drive an electrodelesslamp, such as an electrodeless HID lamp, it is desirable to accuratelydetermine the values of the plasma impedance and the couplingcoefficient between the excitation coil and the lamp. Of course, sincethere are no electrodes, the impedance cannot be directly determinedusing arc voltage and current measurements. Therefore, it is desirableto provide an indirect method for measuring the plasma impedance andfurthermore to provide a simulated load circuit for designing andtesting ballast circuits for electrodeless discharge lamps.

SUMMARY OF THE INVENTION

A simulated load circuit for measuring the impedance of the arcdischarge of an electrodeless discharge lamp of the type having an arctube and an excitation coil for exciting an arc discharge in anionizable fill contained therein comprises: a secondary coil spacedapart from the excitation coil by a distance which is varied in order tovary the coupling coefficient between the secondary coil and theexcitation coil; a fixed load resistance coupled to the secondary coil;and a variable matching network coupled in series or parallel with theload resistance, the impedance of the matching network being varied inorder to vary the ratio of reactance to resistance of the load circuit.According to the present invention, the distance between the secondarycoil and the excitation coil is varied, and the impedance of thematching network is varied, until the input impedance of the simulatedload circuit is substantially equivalent to the impedance of the arcdischarge during lamp operation.

Advantageously, the simulated load circuit described herein is usefulfor designing and testing ballast circuits for electrodeless dischargelamps. Furthermore, the simulated load circuit is useful for providingmeasurements of arc discharge power and excitation coil efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will becomeapparent from the following detailed description of the invention whenread with the accompanying drawings in which:

FIG. 1 schematically illustrates a typical electrodeless HID lampsystem;

FIG. 2 schematically illustrates the equivalent load circuit for thelamp system of FIG. 1;

FIG. 3 schematically illustrates the simulated load circuit of thepresent invention;

FIGS. 4a and 4b schematically illustrate alternative configurations ofthe simulated load circuit of FIG. 3;

FIG. 5a schematically illustrates a preferred implementation of asimulated load circuit of the present invention;

FIG. 5b is a top view of a secondary coil useful in the implementationof FIG. 5a.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an exemplary HID lamp system. (Although the inventionis described herein with reference to an electrodeless HID lamp, it isto be understood that the principles of the invention apply to othertypes of electrodeless lamps, such as electrodeless fluorescent lamps.)As shown, HID lamp 10 includes an arc tube 14 formed of ahigh-temperature glass, such as fused quartz, or an opticallytransparent or translucent ceramic, such as polycrystalline alumina. Arctube 14 contains a fill which may comprise at least one metal halide,such as sodium iodide, and a buffer gas, such as xenon.

Electrical power is applied to the HID lamp by an excitation coil 16disposed about arc tube 14 which is driven by an RF signal via a ballastdriver 18 and a ballast 12. (For clarity of illustration, coil 16 is notshown in its operational position about arc tube 14.) A suitableexcitation coil 16 may comprise, for example, a two-turn coil having aconfiguration such as that described in commonly assigned U.S. Pat. No.5,039,903 of G. A. Farrall, issued Aug. 13, 1991 and incorporated byreference herein. Such a coil configuration results in very highefficiency and causes only minimal blockage of light from the lamp. Theoverall shape of the excitation coil of the Farrall patent is generallythat of a surface formed by rotating a bilaterally symmetrical trapezoidabout a coil center line situated in the same plane as the trapezoid,but which line does not intersect the trapezoid. However, anothersuitable coil configuration is described in commonly assigned U.S. Pat.No. 4,812,702 of J. M. Anderson, issued Mar. 14, 1989, which patent isincorporated by reference herein. In particular, the Anderson patentdescribes a coil having six turns which are arranged to have asubstantially V-shaped cross section on each side of a coil center line.Still another suitable excitation coil may be of solenoidal shape, forexample.

In operation, RF current in coil 16 results in a time-varying magneticfield which produces within arc tube 14 an electric field thatcompletely closes upon itself. Current flows through the fill within arctube 14 as a result of this solenoidal electric field, producing atoroidal arc discharge 20 in arc tube 14. The operation of an exemplaryHID lamp is described in commonly assigned Dakin U.S. Pat. No.4,783,615, issued on Nov. 8, 1988, which is incorporated by referenceherein.

In FIG. 1, ballast 12 is illustrated as comprising a Class-D poweramplifier. However, it is to be understood that the present invention isnot limited to Class-D ballasts, but may apply to any other suitableballast for an electrodeless HID lamp. As shown, ballast 12 includes twoswitching devices Q₁ and Q₂ connected in series with a dc power supplyV_(DD) in a half-bridge configuration. Switching devices Q₁ and Q₂ areillustrated as MOSFET's, but other types of switching devices havingcapacitive gates may be used, such as insulated gate bipolar transistors(IGBT's) or MOS-controlled thyristors (MCT's). Switching devices Q₁ andQ₂ are coupled to ballast driver 18 via input isolation transformers 22and 24, respectively. In operation, the switching devices are drivenalternately between cutoff and saturation such that one is conductingwhile the other one is turned off and vice versa. Hence, the Class-Dballast may be conveniently driven by a square wave signal.Alternatively, ballast driver 18 may comprise means for generating twoout-of-phase sinusoidal signals, as described in commonly assigned U.S.Pat. No. 5,023,566 of S. A. El-Hamamsy and G. Jernakoff, issued Jun. 11,1991 and incorporated by reference herein.

As in any Class-D circuit, a resonant load network is connected to thehalf-bridge at the junction between switching devices Q₁ and Q₂. Such aresonant load network may comprise a series, parallel or series/parallelresonant circuit, depending on the application. In the HID lamp systemillustrated in FIG. 1, the resonant load network includes a seriescapacitor C_(s) which is employed both for resonant circuit tuning andblocking dc voltage. Capacitor C_(s) is connected in series with theparallel combination of the excitation coil 16 of HID lamp 10 and aparallel tuning capacitor C_(p). The parallel combination of capacitorC_(p) and coil 16 functions as an impedance transformer to reflect theimpedance of the arc discharge 20 into the ballast load.

As described in commonly assigned U.S. Pat. No. 5,047,692 of J. C.Borowiec and S. A. El-Hamamsy, issued Sep. 10, 1991 and incorporated byreference herein, capacitors C_(s) and C_(p) are chosen to ensureimpedance matching for maximum efficiency. That is, these capacitors arechosen to ensure that the ballast load is designed for optimum values ofresistance and phase angle. As described hereinabove, the excitationcoil of the HID lamp acts as the primary of a loosely-coupledtransformer, while the arc discharge acts as both a single-turnsecondary and secondary load. The impedance of the arc discharge isreflected to the primary, or excitation coil, side of thisloosely-coupled transformer. To match the ballast load impedance formaximum efficiency, the parallel capacitor operates with the excitationcoil to match the proper resistive load value, and the series capacitoracts with the combination of the excitation coil and parallel capacitorto yield the required phase angle.

FIG. 2 illustrates the equivalent load circuit of the system of FIG. 1.In FIG. 2, R_(c) represents the coil resistance; L_(c) represents thecoil inductance; R_(a) represents the arc resistance; and L_(a)represents the arc inductance. The impedance Z_(L) of the excitationcoil and the reflected arc load are represented as follows: ##EQU1##where k is the coupling coefficient between the excitation coil and thearc discharge; the arc reactance X_(a) =ωL_(a) ; and the coil reactanceX_(c) =ωL_(c), ω being the frequency of operation. Equation (1) may berewritten as: ##EQU2## where Q_(a) =X_(a) /R_(a) is the ratio of arcreactance to arc resistance. From equation (2), it is apparent that todetermine the arc impedance, it is sufficient to determine the couplingcoefficient k and the ratio Q_(a). Advantageously, since only the ratioQ_(a) is required, and not element values, a convenient resistancevalue, e.g., 50 ohms (i.e., the resistance of standard coaxial cables),may be chosen for the simulated load.

FIG. 3 schematically illustrates a simulated load circuit according tothe present invention for measuring the impedance of the arc dischargeof an electrodeless HID lamp. In FIG. 3, L_(s) represents the inductanceof a secondary coil spaced apart from the excitation coil, as describedhereinbelow; Z_(s) =a+jb represents the load impedance as viewed fromthe terminals of the secondary coil; and k' represents the couplingcoefficient between excitation coil L_(c) and the secondary coil L_(s).A measurement block 25 situated between secondary coil L_(s) and theload Z_(s) includes a simple current transformer 27 and a capacitivedivider comprising capacitors C₁ and C₂. Current transformer 27 providesa measure of the load current I_(s) ; and a measurement of the loadvoltage V_(s) is taken across capacitor C₂, as shown. In FIG. 3, thevalues of the elements comprising the measurement block are chosen so asnot to substantially affect the impedance as seen from the secondarycoil L_(s).

According to a preferred embodiment, the impedance Z_(s) includes afixed load resistance, a matching network, and electrical leads. Thematching network may comprise a variable capacitor and/or a variableinductor. Since variable capacitors are more readily available and areeasy to use, the preferred embodiment of the matching network comprisesa variable capacitor. The impedance Z_(L) ' of the simulated loadcircuit may be represented as follows: ##EQU3## where the secondary coilreactance X_(s) =ωL_(s). Equation (3) may be rewritten as follows:##EQU4## where ##EQU5## represents the ratio of reactance to resistanceof the simulated load circuit and Q'_(a) >0.

To achieve impedance matching, the impedance Z_(L) ' of the simulatedload circuit must equal the impedance Z_(L) during lamp operation.Hence, equating the real and imaginary parts, respectively, of equations(2) and (4) results in the following relationships: ##EQU6## where k² >0since the coupling coefficient cannot be complex.

To solve equations (5) and (6), a and b may be determined analyticallyby calculating the impedance of the matching network as viewed from thesecondary coil. Alternatively, a and b may be determined frommeasurements of the magnitude and phase of the current I_(s) and voltageV_(s) at the input to the load Z_(s) (i.e., at the output of coilL_(s)).

FIGS. 4a and 4b illustrate alternative configurations for the simulatedload circuit of the present invention. In FIG. 4a, the secondary coilL_(s) is connected via measurement block 25 and then a first coaxialcable 30 to the matching network (shown as comprising a variablecapacitor C_(v)) which is, in turn, coupled in parallel with the fixedload resistance R_(L) via a second coaxial cable 32. Preferably, thevalue of load resistance R_(L) is equal to the resistance of coaxialcables 30 and 32 in order to provide proper line terminations. In FIG.4b, the secondary coil L_(s) is coupled via two separate coaxial cables30' and 32' to the variable capacitor C_(v) and the fixed resistanceR_(L), respectively. Still other load circuit configurations arepossible. In particular, a load circuit may be constructed whichutilizes a variable resistance in place of fixed resistance R_(L). Foreach load circuit configuration, the impedance as viewed from thesecondary coil L_(s) is different.

FIG. 5a illustrates a preferred implementation of the simulated loadcircuit of the present invention. As shown, secondary coil L_(s) issituated a variable distance d from excitation coil 16. A top view ofsecondary coil L_(s) is shown in FIG. 5b. Secondary coil L_(s) ismounted on an adjustable fixture 33 such that the distance d may bechanged easily. In particular, by changing the distance d between thecoils 16 and L_(s), the coupling coefficient k' therebetween changesaccordingly. Preferably, the fixture has a scale thereon for providingcoupling coefficient values k' corresponding to a range of distances d.The load resistance R_(L) is shown in FIG. 5a as being provided by apower attenuator 34 (e.g. , 50 ohms, -40 dB) . A wattmeter 36, such as,for example, a Hewlett Packard model 435a micro-wattmeter, is connectedto the power attenuator. The variable capacitor C_(v) may comprise, forexample, an air variable capacitor having a range, for example, fromapproximately 5 pF to approximately 1000 pF. The outputs I_(s) and V_(s)from measurement block 25 are appropriately terminated via coaxialcables 40 and 42 at an oscilloscope 44.

In operation, the distance d between the excitation coil and thesecondary coil is varied, and the impedance of the matching capacitorC_(v) is varied, until the impedance of the simulated load circuit issubstantially equivalent to that of the lamp during operation thereof.At that point, the coupling coefficient k' and the ratio Q'_(a) may bedetermined, and thus the corresponding values of the couplingcoefficient k and the ratio Q_(a) are determined from equations (5) and(6) hereinabove.

In particular, the coupling coefficient k' between the secondary coilL_(s) and the excitation coil may be measured by short circuiting thesecondary coil and using the standard formula for the couplingcoefficient, as follows: ##EQU7## where L_(css) represents theinductance of the excitation coil with the secondary coil L_(s)short-circuited. The ratio Q'_(a) may then be derived analytically ormeasured directly as described hereinabove. Substituting into equations(5) and (6) yields the values of k and Q_(a) for the arc discharge.

Advantageously, the simulated load circuit of FIG. 5a can be used tomake other measurements. For example, the power delivered to the arcP_(out) can be measured directly using wattmeter 36. Moreover, thecoupling efficiency of the excitation coil as a ratio of arc powerP_(out) to input power P_(in) can also be accurately determined usingthe simulated load circuit of the present invention.

As another advantage, the simulated load circuit of the presentinvention is useful for testing and tuning HID lamp ballasts duringdevelopment and production. In particular, by using the simulated loadcircuit, it is unnecessary to perform (or even develop) a lamp startingprocedure until after the ballast is tested and tuned.

While the preferred embodiments of the present invention have been shownand described herein, it will be obvious that such embodiments areprovided by way of example only. Numerous variations, changes andsubstitutions will occur to those of skill in the art without departingfrom the invention herein. Accordingly, it is intended that theinvention be limited only by the spirit and scope of the appendedclaims.

What is claimed is:
 1. A method for measuring the impedance of the arcdischarge of an electrodeless discharge lamp of the type having an arctube and an excitation coil for exciting an arc discharge in anionizable fill contained therein when coupled to a radio frequencyballast, said ballast including an impedance matching circuit coupled tosaid excitation coil, comprising:(a) coupling said discharge lamp tosaid ballast and operating said discharge lamp at a predeterminedoperating frequency and tuning said impedance matching circuit to obtaina predetermined ballast load impedance value; (b) disconnecting saiddischarge lamp from said ballast and coupling a simulated load circuitto said ballast for simulating the impedance of the arc discharge ofsaid electrodeless discharge lamp, said simulated load circuitcomprising: a secondary coil spaced apart from said excitation coil suchthat the distance between said secondary coil and said excitation coilis variable, a load resistance electrically connected to said secondarycoil, and a variable matching network electrically connected to saidload resistance; (c) varying the distance between said secondary coiland said excitation coil in order to vary the coupling coefficientbetween said secondary coil and said excitation coil, said simulatedload circuit further comprising a scale for providing a value for saidcoupling coefficient corresponding to the distance between saidsecondary coil and said excitation coil; (d) varying the impedance ofsaid matching network in order to vary the ratio of reactance toresistance of said simulated load circuit; (e) performing steps (c) and(d) until said predetermined ballast load impedance value is obtained,such that the combined impedance of the excitation coil and thesimulated load circuit is substantially equivalent to the combinedimpedance of the excitation coil and the arc discharge when operatingthe lamp; (f) determining said coupling coefficient between saidsecondary coil and said excitation coil from said scale; (g) measuringthe combined impedance of said load resistance and said variablematching network; and (h) calculating the coupling coefficient betweensaid excitation coil and said arc discharge and calculating the ratio ofreactance to resistance of the arc discharge from the measurementsobtained in steps (f) and (g) and thereby determining the impedance ofsaid arc discharge.
 2. The method of claim 1 wherein said variablematching network comprises a variable capacitor.
 3. The method of claim1 wherein said load resistance is fixed.
 4. The method of claim 3wherein said fixed load resistance and said variable matching networkare coupled to said secondary coil via coaxial cable means, said fixedload resistance having substantially the same resistance value as saidcoaxial cable means.
 5. The method of claim 1, further comprising thestep of providing measurement means between said secondary coil and bothsaid load resistance and said variable matching network for measuringthe current supplied to and the voltage across said load circuit.